Many years ago Pioneer 11 flew through Jupiter's
rings, but no one knew it at the time. This week NASA's Galileo
spacecraft did it again ... and scientists were ready.

Jupiter's dark rings--as wide as Saturn's yet nearly invisible--hadn't
been discovered yet. Indeed, it wasn't until five years later
that cameras onboard Voyager 1 caught sight of them for the first
time. On Mar. 5, 1979, the spacecraft swung behind Jupiter, and
from inside the planet's shadow the faintly sunlit rings were
visible--but just barely.

Ever since, researchers have wished for another flyby like
Pioneer 11's. NASA's Voyager, Cassini and Galileo spacecraft
have photographed the rings many times, but always from a distance.
No probe had actually entered the rings for 28 years.

On Nov. 5, 2002, Galileo took the plunge and flew through
Jupiter's rings again. And this time scientists were ready.

"We've been looking forward to this flyby for a while,"
says Joe Burns, a planetary scientist at Cornell University and
a member of the Galileo imaging team. "It's an opportunity
to study the particles that make up these rings and to learn
about their environment."

Galileo is nearing the end of its twice-extended 7-year mission
to Jupiter. Risky maneuvers like flying through Io's volcanoes
and Jupiter's rings were saved for last. This week's ring encounter
and close-approach to Jupiter is one of the final things Galileo
will do before it plunges into Jupiter itself next year.

Unlike Saturn's rings, which are made of bright, icy chunks
as large as houses, Jupiter's rings consist of fine dust akin
to the particles in cigarette smoke. The dust grains are dark
(they reflect barely 5% of the sunlight that hits them) and they
are spread so thin that the rings are almost transparent. This
is what makes the rings so hard to study.

The origin of Jupiter's rings was revealed by Galileo's cameras
more than five years ago. "The dust comes from small rocky
moons orbiting Jupiter," says Burns. These moons are constantly
pelted by meteoroids, which burrow into the ground and explode.
Jupiter's rings are the debris from those impacts.

In fact, Jupiter has several rings: The main ring is
the brightest. It's close to Jupiter and made of dust from the
satellites Adrastea and Metis. Two wide gossamer rings
encircle the main ring. These come from the satellites Thebe
and Amalthea. There is also an extremely tenuous and distant
outer ring that circles Jupiter backwards.
No one is certain, but that ring might be made of captured interplanetary
dust.

When Galileo approached Jupiter last Tuesday, it passed through
one of the gossamer rings. The spacecraft's close approach to
Amalthea on the same day was much-anticipated by scientists who
will figure out the mass of that moon from its gravitational
tug on Galileo.

Right:
A photo of Jupiter's moon Amalthea, which is about as wide as
Long Island. [more]

Saturn's rings probably formed from the total breakup of an
icy moon about the size of Amalthea (100 km wide). Jupiter's
rings, on the other hand, are merely dust from the surface of
such moons. "Saturn's rings are millions of times more massive
than Jupiter's," notes Burns.

Meteoroids have been striking Jupiter's moons and kicking
up dust for billions of years. So why isn't there more "stuff"
in Jupiter's rings? Why are Jupiter's rings so much less massive
than Saturn's?

Burns explains: "Dust grains ejected into Jupiter's rings
don't stay in the rings forever. The grains spiral in toward
Jupiter and eventually disappear."

They lose orbital energy for several reasons: Sunlight is
one. Dust grains absorb and re-emit sunlight, losing momentum
in the process. Scientists call this "Poynting-Robertson
drag."

Plasma collisions are another reason. Jupiter's magnetosphere
(a magnetic bubble that surrounds the planet) is filled with
electrified clouds called plasmas. The dust grains in the rings
are themselves charged--like the static-charged dust that accumulates
on your computer screen. When charged grains collide with plasma
clouds, the grains can lose orbital momentum.

Below: The life of a dust grain in Jupiter's rings.
It begins as debris ejected from a rocky satellite and ends by
spiraling in toward Jupiter. [more]

The
"age" of Jupiter's rings depends on which of these
mechanisms dominates. Plasma collisions might de-orbit ring particles
in only a few years. Poynting-Robertson drag, which Burns favors,
takes longer, perhaps 100,000 years. (The age of Saturn's rings
is likewise controversial. Read Science@NASA's "The
Real Lord of the Rings" for more information.)

Jupiter's rings are constantly replenished by meteoroid impacts,
so they won't disappear any time soon. Next year's rings, however,
might be made of different "stuff" than this year's.
In that sense, Jupiter's rings might be younger than you are.

When Galileo flew through the rings this week, the spacecraft's
suite of electromagnetic sensors and its Dust Detector were working
well. (The spacecraft itself, bombarded by radiation from Jupiter,
went into safe
mode near the end of the ring encounter, but not before data
had been collected.) Burns hopes the unprecedented in situ
measurements will finally solve the puzzle.